FIG. 1A illustrates a conventional semiconductor memory device 5. FIG. 1B is a graph illustrating power consumption curves associated with the storage of different data words in a conventional semiconductor memory device. Referring to FIG. 1A, the semiconductor memory device 5, which may be implemented in a memory card, such as a smart card, includes a central processing unit (CPU) 7 and a memory unit 9.
The CPU 7 outputs an address through an address line B1 and data through a data line B3 when writing data to the memory unit 9. In addition, the CPU 7 outputs an address through the address line B1 and receives data stored at the address in the memory unit 9 when reading the data from the memory unit 9.
During the write operation by the CPU 7, the memory unit 9 receives the address from the CPU 7 and stores the data received through the data line B3 at the address. In addition, during the read operation of the CPU 7, the memory unit 9 receives an address from the CPU 7 and sends the data stored at the address to the CPU 7 through the data line B3.
A conventional semiconductor memory device 5 may be vulnerable to a power attack, during which data stored in the memory unit 9 may be accessed without authorization. A power attack is a method of hacking data by analyzing power consumption occurring when the data is written to an address in the memory unit 9. For instance, when the CPU 7 writes the data word “FF” to an address, power consumption represented by a first curve C1 may occur in the memory unit 9, as illustrated in FIG. 1B. When the CPU 7 writes the data word “00” to the address, power consumption represented by a second curve C3 may occur in the memory unit 9. Accordingly, by analyzing power consumption by the semiconductor memory device, a power attacker can detect that data “FF” and “00” are written to the memory unit 9 when the first and second power consumption curves C1 and C2 occur in the memory unit 9.